The general concept of recovering or recycling particles out of a solution by bubbling the solution so as to float the particles to the surface is well known and used in a wide variety of industries, Commonly known as froth flotation, this is a process whereby hydrophobic particles come in contact with and adhere to bubbles, rising with them to the surface to form a froth as is generally shown at FIG. 1. Publication WO 2014/188232 describes using aerator injection nozzles to recover ore from a slurry or pulp containing water and fine ore particulates. Flotation efficiency has been shown to increase up to 20% percent in the presence of nano/micro bubbles, and while the specific mechanism is not clearly understood this efficiency improvement is considered to be due to nano/micro bubbles. In general for froth flotation separation, the hydrophobic particles bind to the surface of the fine bubbles and float while the hydrophilic particles remain as waste. Various reactants/chemical reagents can be added to the foam or froth flotation system to make the desired particles water repellant so as to bias their attachment to the bubble surfaces. Apart from ore recovery WO 2014/188232 describes how this process can be used to remove paint from recycled paper, oil and fat particles from food industry waste water, particulates in the recovery of contaminated sites, and for treating waste water produced by oil fields.
Further details concerning nano-bubble froth flotation industrial processes for efficient recovery of fine and course mineral, coal, oil sand and paper particles in mining and recycling industries can be seen in a document by S. Ahmed entitled Cavitation nanobubbles enhanced flotation process for more efficient coal recovery (PhD thesis, Kentucky University, 2013). Estimating adhesion of the bubbles to particle surfaces from contact angle measurements is explored in an article by S. R. Rao entitled Surface Chemistry of Froth Flotation (SPRINGER, Vol. 2, 2004). The above-referenced PhD thesis by S. Ahmed asserts that large-particle recovery technology is currently limited to about 100 micro-meters for minerals and 600 micro-meters for coal, but an article by A. Goodbody entitled Staying the Coarse (MINING MAGAZINE, November 2014) proposes that enabling flotation of coarse particles has the potential to reduce the energy in the froth flotation process chain with significant economic benefits.
Nano/micro-bubbles form naturally or can be generated artificially. Systems for generating nano and/or micro-bubbles are proposed in various publications including U.S. Pat. No. 7,591,452 and US patent application publication no. 2007/0189972; as well as papers by Z. Zhou, H. Hussein, Z. Xu, J. Czarnecki and J. H. Masliyah entitled Interaction of ionic species and fine solids with low energy hydrophobic surface from contact Angle measurement (JOURNAL OF COLLOID INTERF. SCI., Vol. 204, pp. 342-349, 1998); by R. Clift, J. R. Grace and M. E. Weber entitled Bubbles, drops and particles (ACADEMIC PRESS, 1978); and by H. Takushoku entitled Progress in Chemical Engineering—Bubble, Drop, and Dispersion Engineering (MAKI SHOTEN, 1982). The majority of these techniques are seen to generate nano-bubbles with a non-uniform size distribution, which can be fitted by a normal distribution. It appears to the inventors that the only known technique that produces micro/nano-bubbles with a relatively uniform size distribution is described in U.S. Pat. No. 7,591,452, which injects gas through a porous materials having similar-diameter pores. After the nano/micro-bubbles are injected they attach to particles or stay dispersed in the solution.
There are known methods used to provide a digital image of the nano/micro-bubbles under investigation. They include neutron reflectometry (Steitz, R., Gutberlet, T., Hauss, T., Klosgen, B., Krastev, R., Schemmel, S., Simonsen, A. C. and Findenegg, G. H., Nanobubbles and Their Precursor Layer at the Interface of Water Against a Hydrophobic Substrate, LANGMUIR, Vol. 19, pp. 2409-2418, 2003), X-ray reflectivity measurements (Poynor, A., Hong, L., Robinson, I. K., Granick, S., Zhang, Z. and Fenter, P. A., How Water Meets a Hydrophobic Surface, PHYS. REV. LETT. Vol. 97, pp. 266101-266104, 2006), optical spectroscopy (Zhang, X. H., Khan, A., and Ducker, W. A., A Nanoscale Gas State, PHYS. REV. LETT. Vol. 98, pp. 136101-136104, 2007), optical microscopy (Karpitschka, S., Dietrich, E., Seddon, J. R. T., Zandvliet, H. J. W., Lohse, D. and Riegler, H., Nonintrusive Optical Visualization of Surface Nanobubbles, PHYS. REV. LETT. Vol. 109, pp. 066102-066105, 2012) and tapping mode atomic force microscopy (Wang, Y., and Shushan, B., Boundary slip and nanobubble study in micro/nanofluidics using atomic force microscopy, SOFT MATTER, Vol. 6, pp. 29-66, 2010).
Previous computer vision studies in the flotation industry involved capturing images to assess froth characteristics such as mass, color and flow rate (Francois, E. Du P. and Marc, V. Olst, Monitoring and control of a froth flotation plant, 2004; and also international patent application publication WO 1997/045203), as well as particles and bubbles size, distribution and concentration (WO 1997/045203). All these parameters in the flotation cell are directly related with the grade and recovery efficiency of the foam/froth flotation system.